There is a class of semiconductor products that are predominantly planar and specular (flat and shiny). It is frequently necessary or desirable to image these devices in such way that even minor deviations from planar and specular are imaged with adequate contrast. One such class of products are semiconductor wafers that may be provided with indicia that indicate, among other things, wafer number and manufacturer. These indicia are defects in the surface of the wafer and are typically a matrix of laser etched pits. These indicia are known in the art as “soft marks.” These marks are imaged to read the codes at various steps along the manufacturing process.
After the semiconductor wafers have been singulated (generally cut by saw and/or laser into individual rectangular devices), it may be necessary or desirable to inspect the edges for small chips and cracks that may propagate over time and cause premature device failure. These inspection processes are automated and use electronic imaging cameras in combination with digital electronic computers that are programmed to perform the necessary inspections, measurements and identifications.
Dark field lighting, in general, is a technique well known to those skilled in the art and is particularly useful to inspect defects on specular objects. The definition of dark field lighting is dependent upon the properties of the illumination source, its position relative to both the object and the observer, or camera, and on the properties of the object being illuminated. In order to meet the definition of dark field lighting, it is necessary that the majority of the illumination incident on the object is reflected in a direction or directions that do not enter the optical aperture of the observer or camera. Dark field illumination can be compared against bright field illumination where the majority of light is reflected directly into the camera.
Dark field lighting can be achieved by placing a light source such that it is pointing at the object at an angle to the line between the camera and the object. This angle is selected to be greater than the angle over which the object will diffuse light. If the object has a generally diffuse reflective nature, then the angle is selected to be larger than the half-angle over which the object will distribute incident illumination by diffuse reflection. If the object is specular (e.g., if the object diffuses incident illumination over a small angle, or with very low efficiency, or both), then the angle may be selected to be very small.
It may be desirable to make the illumination source symmetric. In this case, the source may be manufactured in an annular shape and placed coaxial to the optical axis, or a plurality of sources may be arranged in an annular shape. The diameter of this annulus and its proximity to the object determine the range of angles over which the illumination is incident upon the object. Such lights are known to those skilled in the art as ring lights and are variously configured to be “high angle” or “low angle.”
In imaging certain objects, it is desirable to highlight very minor features in a surface which is otherwise substantially planar and specular. These include soft marks and the edges of singulated devices. To achieve this, it is necessary to bring the illumination source as nearly on-axis with the imaging system as possible without causing the illumination source to be directly reflected into the imaging system (e.g., selecting a narrow angle). An effective way to achieve this, as currently known, is with the aid of baffles and providing a particular alignment between the illumination source, the object, the baffles, and the imaging system.
In the design of machine vision systems and wafer identification (ID) systems, a designer (e.g., systems engineer) generally makes a number of design compromises. For example, if a lens aperture is made large, a limiting resolution of the system will generally be higher and the system will generally be more efficient. A more efficient system demands less sensitivity or gain from an image sensor, and the system demands less light from an illumination system. If the illumination system demands less light, less demand is placed on a power supply system and less heat is dissipated. Heat dissipation is generally a major hurdle in achieving a design objective of achieving compact packaging.
Conversely, if the system aperture is made small, the limiting resolution of the system is reduced, some aberrations are reduced, a depth of focus is increased, a very significant demand is made upon the image sensor for sensitivity and/or gain, and a very significant demand is placed on the illumination system to provide substantially more light. Demands on the illumination system place demands upon the power supply system and thereby exacerbate problems of thermal dissipation within a compact package.
Heretofore, commercial wafer ID reading systems, including previous generations manufactured by Electro Scientific Industries, Inc. of Portland, Oreg., the assignee of this patent application (e.g., ScribeView™ Models 1 through 5P) and others within the industry (e.g., Cognex Corp. of Natick, Mass., and Kowa Co., LTD. of Tokyo, Japan) have employed optical systems with a working distance range (depth of focus in object space) of approximately ±1 mm or less. Although such systems are workable, they require a user to make adjustments to the focus position of the lens and/or an adjustment to the position of the wafer ID reader to account for even minor changes to the working distance. The working distance may change, for example, if the thickness of the object changes, or if there is imprecision in the robotic system that presents the wafer to the wafer ID reading system.
It would be preferable if the range of working distances for a wafer ID system in a fixed configuration spanned more than approximately ±1 mm. If this range could be extended by an order of magnitude, focusing and setup of the system would be trivial compared to existing systems. For example, current systems generally require mounting and focusing of the system in a powered-up state in accordance with an iterative procedure in which the image can be electronically monitored and a user may make a change in focus adjustment or position while monitoring an electronically presented image. The user generally makes adjustments until a satisfactory image is obtained. If the working distance range could be extended to approximately ±10 mm, the wafer could be mounted while the wafer ID system is in an unpowered state using a simple ruler to determine an appropriate working distance.
Moreover, with existing systems, process variations that cause a change in working distance of approximately 1 mm generally require user intervention to either adjust the process to re-achieve the previous working distance or to adjust the focus of the wafer ID system, for example, by turning a lens-focus barrel or turning a focus-adjustment screw. It would be preferable if minor process variations that resulted in changes in the working distance by just a few millimeters could be accommodated by the wafer ID system with no user intervention.